command button to clear text box AND ad value

Hello,

Userform1 has 3 command buttons and 3 text boxes. A mouse click on a text box followed by a command button click a value of 1, 2, or 3, respectively, is entered in the text box. This is the code:

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The condition and action I want to add to the code is:
1. Only one command button value is permissible in a text box
2. If a text box contains a value, pressing the command button will clear the text box and
2a. enter the command button value.

Any thoughts on how I can write this in to my existing code?

Thank you in advance.
Gal403